1. Field of the Invention
The present invention relates to a fuel-cells generator system using fuel cells that receive a supply of a reaction gas fed to an electrode with a catalyst carried thereon and generate an electromotive force through a chemical reaction of the reaction gas, and also to a method of generating electricity from fuel cells.
2. Description of the Related Art
Fuel cells are a known device for directly converting chemical energy of a fuel to electrical energy. Each fuel cell includes a pair of electrodes arranged across an electrolyte, wherein the surface of one electrode is exposed to hydrogen or a hydrogen-containing gaseous fuel and the surface of the other electrode is exposed to an oxygen-containing, oxidizing gas. Electrical energy is taken out of the electrodes through electrochemical reactions.
As is known, the output of electrical energy from the fuel cells depends upon a variety of driving conditions, such as a gas pressure, a cell temperature, and a gas utilization ratio. Conventional structure enhances the output of the fuel cells by appropriately regulating these driving conditions. An example of the conventional structure is a fuel-cells generator system disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 5-283091. This system controls the driving temperature of the fuel cells to an ideal operating temperature (approximately 80.degree. C. in the case of polymer electrolyte fuel cells) so as to enhance the battery output.
In case that the catalyst carried on the electrode of the fuel cells is poisoned by carbon monoxide, the control of the driving temperature of the fuel cells to the ideal operating temperature may not result in high output from the fuel cells. The applicant of the present invention has accordingly proposed an improved fuel-cells generator system disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 8-138710. The proposed system controls the driving temperature of the fuel cells to a predetermined temperature higher than the ideal operating temperature, thereby enhancing the output of the fuel cells even in the poisoned state of the catalyst on the electrode.
The control of the driving temperature of the fuel cells to be higher than the ideal operating temperature in the poisoned state of the catalyst on the electrode enhances the output of the fuel cells, because of the following reason. The equilibrium of adsorption and release of carbon monoxide on and from the surface of the platinum catalyst carried on the electrode in the fuel cells is shifted to the direction of releasing carbon monoxide with an increase in temperature of the fuel cells. This means that the amount of adsorption of carbon monoxide decreases with an increase in temperature of the fuel cells. When the temperature of the fuel cells becomes higher than the ideal operating temperature, the degree of this effect cancels the decrease in battery output due to the increased temperature of the fuel cells. In the poisoned state of the catalyst, the increased temperature of the fuel cells to be higher than the ideal operating temperature thus enhances the battery output.
As mentioned above, when a decrease in battery output is detected in the poisoned state of the catalyst on the electrode, the control of the driving temperature of the fuel cells to a predetermined temperature higher than the ideal operating temperature can enhance the battery output.
The prior art technique, however, can not sufficiently enhance the battery output when the temperature of the fuel cells is too high in the poisoned state of the catalyst on the electrode. In the case of polymer electrolyte fuel cells, the ideal operating temperature is approximately 80.degree. C. In the prior art technique, the fuel cells are accordingly driven at the higher temperatures of 90.degree. C. to 95.degree. C. In case that the fuel cells are driven in a still higher temperature range, the reaction substance included in the gas, that is, hydrogen on the anode and oxygen on the cathode, can not be sufficiently supplied to the reaction interface of each electrode or more precisely to the surface of the catalyst. This prevents the fuel cells from being driven stably to give the high output.